1. Field of the Invention
The present invention relates to a receiver capable of outputting a high quality signal without regard to the level of an input signal. More particularly, the present invention relates to, e.g., an optical receiver included in an optical communication system and capable of amplifying the level of a signal derived from a received optical signal to a desired level. The receiver of the present invention is adaptive even to burst signals and may handle electric signals in place of optical signals.
2. Description of the Background Art
An optical receiver, for example, includes a light-sensitive device for converting a received optical signal to an electric signal and amplifies the electric signal. A prerequisite with the optical receiver is that its output includes a minimum of noise and has a broad dynamic range, a broad frequency band, and a high gain. Another prerequisite is that the receiver can be easily interfaced to a device to follow.
Japanese patent laid-open publication No. 200709/1986, for example, discloses a pre-amplifier for optical communication as an amplifier meeting the above prerequisites. In this pre-amplifier, a differential amplifier has one of its two input terminals connected to the output of a transimpedance amplifier that amplifies the output of a light-sensitive device. The other input of the differential amplifier is connected to the output of a reference voltage generating circuit identical in configuration with the transimpedance amplifier with respect to DC. The pre-amplifier with such a construction and connection is applied to, e.g., an optical receiver for an optical communication system.
In an optical communication system, a transmitter of the type directly modulating the intensity of light in accordance with data and sending the data in the form of a pulse beam is conventional. This type of transmitter transforms an input electric pulse signal to an optical pulse signal or pulse beam.
Assume that the transmitter outputs an optical signal P.sub.s when a pulse signal representative of data is present or outputs an optical signal P.sub.b when it is absent. The optical signal P.sub.b is derived from, e.g., a dark current flowing through a light emitting section and representative of the DC offset component of a signal. The signal P.sub.s contains the signal P.sub.b. The signal output from the transmitter has a characteristic determined in terms of, e.g., a ratio of the signal P.sub.s to the signal P.sub.b generally referred to as an extinction ratio. In the optical communication system, optical signals with such a characteristic are interchanged. The transmission system required to send and receive high quality signals at a high speed is usually constructed to maintain the offset component small enough to guarantee acceptable transmission quality.
When the pre-amplifier mentioned earlier is applied to a low cost, simple optical communication system, the above extinction ratio directly appears in the received signal as an offset voltage input to the differential amplifier. Moreover, when a differential amplifier with a high output gain is used, and if an input signal has an extremely high level, the amplifier fails to linearly amplify the input signal and thereby limits the output amplitude, i.e., causes the output to saturate, as well known in the art. It follows that the pre-amplifier received the signal in the above condition undesirably raises the lower limit, i.e., DC voltage level by amplification due to the input offset voltage. Consequently, the saturation level limits the upper limit of the amplitude level and thereby causes the amplitude range of the output signal to be practically lost.
Further, when the differential amplifier has a low gain and receives an input signal having an extremely low level, it cannot amplify the input signal to a sufficient degree. In this manner, the dynamic range of the output signal is reduced when the input signal has an extremely high level or an extremely low level.
In light of the above, a differential amplifier for the above application is usually implemented by a variable-gain amplifier capable of varying its gain in accordance with the level of an input signal. Specifically, a variable-gain amplifier has its gain reduced when the input signal has an excessively high level or has the gain increased when it has an excessively low level, thereby guaranteeing a linear dynamic range. To set the gain of the variable-gain amplifier, an input signal is first sampled in order to hold and determine the peak of the signal. Then, a gain control signal for optimizing the dynamic range on the basis of the peak is fed to the amplifier in the form of a DC voltage. As a result, the gain of the amplifier is automatically controlled (so-called automatic gain control or AGC).
However, the above AGC processing is not practicable unless it samples the continuously input signal for a certain period of time. The AGC processing is therefore not feasible for an optical communication system involving a time constant, e.g., PDS (Passive Double System) or PON (Passive Optical Network) handling burst signals which must not lose any data. This is because a receiver included in this kind of system must accurately receive all the data without any loss despite that signals are irregularly input to the variable-gain amplifier as to timing and have different levels.
In recent years, optical transmission dealing with burst signals has been studied and disclosed in, e.g., R. G. Swartz et al. "Electronics for High Speed, Burst Mode Optical Communications", International Journal of High Speed Electronics, Vol. 1, Nos. 3 & 4, pp. 223-243, 1990. This document describes a concept relating to the burst mode of an optical communication system as well as differences particular thereto, and proposes a new solution using high speed, high accuracy peak detection.
Another problem with the pre-amplifier for optical communications is that its output signal is distorted when ringing occurs in the transimpedance amplifier. Digital data produced from the distorted signal would include bit errors.
Moreover, the sensitivity of the pre-amplifier is determined by a noise current in terms of an input. To produce a noise current in terms of an input, the noise current of a reference voltage generator built in the pre-amplifier and that of the transimpedance amplifier each are squared and then averaged, and the resulting mean squares are summed. It follows that sensitivity available with the pre-amplifier is too low to realize a high sensitivity optical receiver.